The present invention relates to an apparatus for driving an ac motor which runs by switching the ac motor driven by an independent ac power supply to the side of a current controlled type power converter which produces an ac current or power having an arbitrary frequency as its output.
For apparatus widely used in general as current controlled type power converters to which the present invention is applied, current type inverters or thyristor motors etc. are known. By giving an example of the current type inverter as the current controlled type power converter, the following discussion will proceed. In addition, induction motors and synchronous motors etc. are applicable to the ac motor. In the following discussion, explanation will be made by giving an example of the induction motor as the ac motor.
Induction motors are driven in accordance with various running systems using various power supplies depending upon their applications. For instance, when applied to a fan or blower etc., there are many instances where the induction motor runs at a constant speed using a commercial power supply. In an attempt to perform a flow rate control in such a system, control using a damper etc. has been generally conducted. To the contrary, there have been recently increased applications to effect an adjustable speed control using the current type inverter, thereby to perform the flow rate control. In accordance with this method, when it is required to cause the flow rate to be maximum, running is carried out by directly using the commercial power supply without connecting the current type inverter to the system, resulting in extremely high reliability and good efficiency. On the other hand, when the flow rate is required to be reduced, since the induction motor is driven through the current type inverter using a running frequency caused to be reduced due to the connection of the inverter to the system, such a control advantageously provides that running with a high efficiency and a saving in energy is possible when compared to the above-mentioned damper control.
In FIG. 5, the above-mentioned control system is shown. A commercial power supply 11 is connected to a current controlled inverter through a switch 12 and a transformer 13. The current type inverter is composed of a rectifying circuit 14 which converts ac to dc current or power, a dc reactor 15 which smooths the dc current from the circuit 14, and an inverter circuit 16 which converts dc to ac current or power having an arbitrary frequency. The output of the current type inverter is connected to an induction motor 18 through a switch 17. The commercial power supply 11 is also connected via a different route directly to the induction motor 18 through a switch 19. The current type inverter operates as will be described below. Namely, a speed reference given by a setter 21 serves as a voltage reference e.sub.v and a frequency reference e.sub.f through an input limiter 22. The voltage reference e.sub.v and a voltage feedback signal obtained through a volatage transformer 23 and a rectifier 24 are subjected to comparative amplification at a voltage control circuit (VC) 25 to produce a current reference. This current reference and a current feedback signal obtained through a current transformer 26 and a rectifier 27 are subjected to comparative amplification at a current control circuit (CC) 28 to produce a phase reference. This phase reference gives a firing pulse of thyristors constituting the rectifying circuit 14 through a phase control circuit (PHC) 29. On the other hand, the frequency reference e.sub.f gives a firing timing of thyristors constituting the inverter circuit 16 through an oscillator (OSC) 30 and a ring counter (RING) 31. A pulse amplifier circuit (PA) 32 between the ring counter 31 and the inverter circuit 16 has a function to amplify the firing pulse, and a function to determine whether or not a pluse is delivered to the thyristors constituting the inverter circuit 16 in accordance with a running singal RUN externally delivered. Switching from the drive by the commercial power supply to the drive by the current type inverter is carried out as follows. Until such a switching is effected, the switch 19 is closed, and the phase of a current supplied from the commercial power supply is detected by a current transformer 33 and a current detection circuit (CD) 34. The reason why the detection of the current phase is carried out is that the power converter to be switched is of the current type and therefore the factor which can be directly controlled is the current phase. The current phase output from the current detection crcuit 34 and the firing timing output from the ring counter 31 are subjected to comparative amplificaiton in terms of phase at a phase comparator (PLL) 35. Thus, the phase comparator 35 produces to the oscillator 30 a frequency correction signal .DELTA.e.sub.f for allowing both frequencies and phases to be equal to each other. In a manner stated above, before the running signal RUN is input, the current type inverter is caused to be in phase with the side of the commercial power supply, thereafter to initiate running of the current type inverter. In addition, outputs from the current detection circuit 34 and the ring counter 31 are delivered to a synchronization detector (SYD) 46, and the output of the SYD 46 is used for control of the operation of the switch 17.
The operation of the synchronization detector 46 shown in FIG. 5 will now be described with reference to FIG. 7. When an output signal e.sub.a of the current detection circuit 34 is input to the synchronization detector 46, a monomultivibrator 46a responds to this signal e.sub.a to produce a predetermined pulse signal which is input to an AND gate 46c. Likewise, when an output signal e.sub.b of the ring counter 31 is input to the synchronization detector 46, another monomultivibrator 46b responds to this signal e.sub.b to produce a predetermined pulse signal which is input to the AND gate 46c. When the AND condition on both pulse signals holds at the AND circuit 46c, this circuit 46c produces an input signal to a counter circuit 46d. When such signals are successively input to the counter circuit 46d by a predetermined number, the counter circuit 46d outputs a synchronization detection signal. In contrast, when such signals are not successively input, a counter 46e responds to the number of pulse signals output from the monomultivibrator 46a to produce an input signal to an AND circuit 46f. A signal obtained by inverting an output signal from the counter circuit 46d using a NOT circuit 46g is also input to the AND circuit 46f. At this time, if the counter circuit 46d does not effect synchronization detection, the counter circuit 46d and the counter 46e are both reset.
FIG. 6 illustrates an example of a main circuit configuration of the current type inverter. The rectifying circuit 14 is composed of six thyristors. In addition, the inverter circuit 16 is composed of six thyristors, six diodes and six commutation capacitors. Since the detailed operation thereof is not significant for the subject matter of the present invention, the explanation thereof will be omitted. It is to be noted that the detailed operation is described, for example, in "Running and maintenance of thyristor controlled ac motor" by Naohiko Yamagami (published by Denki Shoin, Oct. 29, 1977).
In accordance with the conventional power supply switching system shown in FIG. 5, while the system is subject to switchover running, i.e., when the switches 19 and 17 are closed at the same time, the current phase on the side of the commercial power supply which serves as reference shifts so as to correct the phase of the current from the current type inverter side. Thus, the drawback with this conventional system is that the reference phase is not fixed, resulting in unstable switchover running. In addition, two or three current transformers are required for accurately detecting the current phase on the side of the commercial power supply. Thus, further drawback is that when applied to the high voltage system, the entirety of the system becomes costly because the transformer for high voltage is expensive.